hi mate i act just bought some new guylines today from decathlon, they do not glow in the dark or anything but are a lot brighter than stock, i think they are in the diving section and they are £5 for 20 meters and i get the 3mm dia one and there are loads of colours to choose from and it is very good quality line. i have used it on three of my tents and never had a problem with it yet.
hope this helps Winnie